Currently, mobile terminals and mobile networks using wideband code division multiple access (W-CDMA) technology of the third generation realize voice calls based on a circuit-switched (CS) system. Meanwhile, in response to the trend of mobile core networks toward use of Internet protocol (IP), circuit switched over IP network (CSIP) is appearing. Based on CSIP, a circuit-switched signal is converted into an IP signal, and the IP signal is then connected to an IP multimedia subsystem (IMS) network. In the circuit-switched network, an integrated services digital network (ISDN) user part (ISUP) call control signal and an audio compressed-and-encoded bit stream are transmitted and received by an asynchronous transmission mode (ATM) bearer. However, in the IMS network, a session initiation protocol (SIP) signal is used for session control, and an audio compressed-and-encoded bit stream is transmitted and received after stored in a real-time transport protocol (RTP) packet.
It is conceivable that mobile networks will be further improved in speed and capacity through research and development, based on IP. In fact, high speed downlink packet access (HSDPA), high speed packet access, HSPA+, evolution-data optimized (only) Rev.A (EVDO Rev. A), and EVDO Rev.B have already been in practical use as mobile high-speed networks.
In addition, long term evolution (LTE) aiming at 100 Mbps (Megabit per second) or greater in the downlink direction and 50 Mbps or greater in the uplink direction and evolved packet core (EPC) as an IP backbone network supporting LTE are expected to be introduced as mobile high-speed networks.
Audio communication needs to be continuously supported for mobile terminals connected to these LTE and EPC. In LTE and EPC, RTP packets are used to execute voice over IP (VoIP) audio communication. In addition, the LTE section is expected to include a function of changing an audio encoding bit rate when network congestion is detected.
Patent Document 1 discloses a delivery server apparatus converting stream media data into a false stream and delivering the false stream, to provide mobile communication terminals, without inconvenience, with information delivery services using resource description framework (RDF) site summary (RSS) feed for personal computer terminals.
In addition, Patent Document 2 discloses a stream delivery apparatus adjusting a dynamic transmission rate while maintaining a real time property and quality as much as possible, even when the load of a network or a system is fluctuated.    Patent Document 1: Japanese Patent Kokai Publication No. 2007-259128A    Patent Document 2: Japanese Patent Kokai Publication No. JP-H11-341063A